Photochemical Oxygen Consumption Sensitized by a Porphyrin Phosphorescent Probe in Two Model Systems

AbstractPhosphorescence quenching of certain metalloporphyrins is used to measure tissue and microvascular pO2. Oxygen quenching of metalloporphyrin triplet states creates singlet oxygen, which is highly reactive in biological systems, and these oxygen-consuming reactions are capable of perturbing tissue oxygenation. Kinetics of photochemical oxygen consumption were measured for a Pd-porphyrin in two model systems in vitro over a range of irradiances (1.34–134mWcm−2). For a given irradiance, and, after correction for differing porphyrin concentrations, rates of oxygen consumption were similar when the Pd-porphyrin was bound to bovine serum albumin and when it was taken up by tumor cells in spheroids. At irradiances comparable to those used in imaging superficial anatomy, rates of oxygen consumption were sufficiently low (2.5μMs−1) that tissue oxygenation would be reduced by a maximum of 6%. An irradiance of 20mWcm−2, however, initiated a rate of oxygen consumption capable of reducing tissue pO2 by at least 20–40%. These measured rates of consumption impose limitations on the use of phosphorescence quenching in thick tissues. The irreversible photobleaching of the Pd-porphyrin was also measured indirectly. The bleaching branching ratio, 23M−1, is significantly lower than that of porphyrin photodynamic agents